We have found bacterial mutants that are highly sensitive to visible and near-ultraviolet light and to a tryptophan photoproduct (TP). These mutants also lack the ability to undergo genetic recombination (i.e., they are recombinationless mutants). In addition to this lethal effect, mutations are also produced by these agents. The tryptophan photoproduct interferes with the process of genetic recombination. The broad objectives of this proposal are: (1) to complete the separation and identification of the tyrptophan photoproduct that causes these biological damages. Also, to separate and identify any other photo-receptors (chromophore) that load to alterations in the DNA of bacterial cells; (2) to identify the macromolecule with which tryptophan photoproduct interacts (i.e., DNA, one of the repair enzymes, primer RNA, etc.); (3) to determine the specific manner whereby TP interferes with genetic recombination, DNA repair and replication; (4) to test whether TP interferes with DNA replication and repair in eukaryotic cells in the same manner as it does in bacterial cells; (5) to identify the specific molecular alterations that account for mutation by light (above 300 nm) and by tryptophan photoproduct (i.e., base pair change, transition, transversion, frameshift, deletion); and (6) to test the abiotic effects of TP for other biological phenomena; e.g., cell division and differentiation in sea urchin eggs, plant development, photo-effects in insects and other small animals.